1. Field of the Invention
The present invention relates generally to microwave antennas.
2. Description of the Related Art
There is a growing commercial denhand for low-cost radar systems. For example, investigators around the world are working on the development of collision-avoidance radar systems for use in automobiles, trucks, boats and small aircraft. A key element of these radar systems is an antenna that can radiate a scanned microwave beam. Obstacles that are interrogated by the scanned beam cause an echo which is received by the antenna and sent to an electronic portion of the radar for processing.
If a collision-avoidance radar is to be commercially viable, its elements, such as the scanned antenna, must be light weight, low cost, spatially compact and offer good performance with low maintenance costs over a long lifetime (e.g., &gt;10 years). In addition, the scanned antenna should preferably be based on technologies that are well developed so as to reduce technical and schedule risks.
Apparatus for scanning a microwave antenna beam have generally fallen into two groups, mechanically-scanned antennas and electronically-scanned antennas. Gimbal systems have been extensively used in aircraft to facilitate the mechanical scanning of fixed-beam antennas. However, gimbal systems are typically heavy and costly to ihbricate and usually require considerable maintenance.
Electronic scanning has often achieved high peribrmance but at the cost of complexity, weight and cost. For example, antennas have incorporated movable waveguide vanes which vary the phase of radiation through waveguide slots (e.g., see Markus, John, et al., McGraw-Hill Electroncis Dictionary, McGraw-Hill, New York, 5th Edition, 1994, p. 390). These systems involve a large number of moving parts so that both fabrication and maintenance costs tend to be high. Phased array antennas typically employ a plurality of phase shifters, e.g., ferrite and electronic, to provide beam steering (e.g., see Stimson, George W., Introduction to Airborne Radar, Hughes Aircraft Company, El Segundo, 1983, pp. 577-580). Phased arrays can achieve high-speed scanning but the phase shifters and associated parts, e.g., waveguide networks and amplifiers, result in complex fabrication and high parts count.